The present invention relates to a double-progressive spectacle lens.
Single-progressive spectacle lenses and also double-progressive spectacle lenses are known from the prior art. For example, U.S. Pat. No. 6,089,713 (=DE 197 01 312) describes a spectacle lens having a spherical front side and multifocal rear side, and also a method for producing it. Published German patent application no. DE 33 31 757 A1 describes a progressive spectacle lens having a convex aspheric front surface and concave aspheric inside or rear surface.
However, it has been found in the case of spectacle lenses having progressive rear surfaces that in the stronger addition area the spherical front surfaces must be fashioned with more of a curve than the front surfaces in the distance area of lenses having progressive front surfaces. This is felt to be disadvantageous by the spectacle wearer both for optical and for cosmetic reasons. The reason for this resides in the geometry of the lenses: if, for example, a lens is designed with a distance area effect of +5 dpt, in the case of progressive front surfaces use is made, for example, of a front surface with a surface dioptric power of +7 dpt which rises in the near area to approximately +10 dpt if the lens is to have an addition of 3 dpt. The spherical rear surface then has a surface dioptric power of approximately −2 dpt, resulting in an effect of approximately +5 dpt in the distance area.
If, in the case of progressive rear surfaces, a spherical front surface is to be selected which is as flat as the distance area of the lens with a progressive front surface (that is to say 7 dpt), the rear surface would need to have a surface dioptric power of −2 dpt in the distance area. With progressive rear surfaces, the mean curvature needs to decrease from the distance area to the near area and not, however, to increase such as would happen with progressive front surfaces. Thus, the surface dioptric power must rise from −2 dpt by 3 dpt to +1 dpt, in order to effect an addition of 3 dpt. This form of surface configuration is termed reversal of curvature. The surface is no longer only concave, as is the case with conventional products, but has an S-shaped course in a vertical section.
However, rear surfaces with reversal of curvature exhibit specific wearing properties which can lead to problems for the spectacle wearer. One problem is the relative nearness of the rear surface to the eye when such lenses are countersunk in conventional mounts. Since the eyelashes then strike against the lens, this frequently leads to irritation. A further problem is the more frequently occurring reflections which have a disturbing effect and reduce the contrast during seeing.
In order to avoid these effects, it is possible to increase only the curvature of the front surface in the case of lenses with a progressive rear surface, but this entails lenses which are thicker and unattractive cosmetically. Again, this raises the intrinsic magnification of the lenses, the result of which is to intensify the so-called “cow-eye effect”.